Summary Sudden cardiac death kills 180,000 to 450,000 Americans annually. In patients without coronary artery disease, the highest risk of sudden death is in those with cardiomyopathy and intraventricular conduction delay. Furthermore, males experience sudden death almost three times more frequently than females and the reasons for this sex bias are unclear. Identifying the mechanisms underlying cardiomyopathy and conduction disorders may improve screening for patients at risk for sudden death, or outcomes of those revived from cardiac arrest. To date, most known mutations that cause cardiomyopathy are in sarcomere cytoskeletal proteins whereas most familial cardiac conduction diseases (CCD) are caused by mutations in ion channels or channel interacting proteins. However, mutations in cytoskeletal proteins can also cause CCD but the mechanisms linking the cytoskeleton to CCD are not well understood. There is a gene or genes on chromosome 6 (6p22) associated with sudden cardiac death near the cytoskeletal protein CAP2 (Cyclase Associated Protein 2). CAPs are widely conserved cytoskeleton proteins. We first identified CAP in yeast as an adenylyl cyclase binding protein. Additionally, all CAP homologs are actin monomer binding proteins that regulate the balance between actin filaments and actin monomers. Mammals have two CAP isoforms, CAP1 and CAP2. To determine the function of CAP2 in vivo we generated CAP2 knockout (CAP2-KO) mice, both whole body and conditional. Our preliminary data show that whole body CAP2-KO mice are born alive, but many die suddenly shortly after birth, with only ~30% of male CAP2-KO surviving beyond 12 weeks. CAP2-KO mice develop CCD with mild dilated cardiomyopathy (DCM). The conduction phenotypes are more penetrant in cardiomyocyte-specific CAP2-KO mice, with all mice dying of complete heart block by 25 weeks. Furthermore, genome-wide analysis revealed that several transcriptional networks, including the serum response factor (SRF) network, were upregulated in the hearts of CAP2-KO mice. Based on these findings we hypothesize CAP2 maintains cardiac conduction by modulating SRF signals and fibrotic responses in a sex-specific manner. These studies are relevant to understanding the role of the cytoskeleton and gender specificity in cardiac conduction and sudden cardiac death. To test this hypothesis, we propose the following specific aims: To test this hypothesis, we propose to (1) Determine the specific role of CAP2 in the cardiac conduction system (2) Determine the role of CAP2 in serum response factor (SRF) signaling, and (3) Determine the role of Secreted Frizzled-related protein 2 in sex-specific sudden cardiac death in CAP2 mice.